Experimental studies on the calibration of actual organ size by computed tomography

Examinations were carried out on the stability of the computed tomographic (CT, Delta) numbers and on the method of volume calibration. The diurnal and daily changes and spatial independence of the Delta numbers were slight even though some visible artefacts were present. At the interface of the two materials of different x-ray attenuation coefficient, Delta numbers change gradually from that of one material to that of the other. This makes it inaccurate to calibrate the actual size of materials by CT images. This phenomenon is chiefly due to the partial volume effect and the edge response, although even in this instance, the true boundaries of materials can be identified with the median Delta number of the two substances.